Radio communication system



D. H. HUGHES RADIO COMMUNICATION SYSTEM Filed July 26, 1947 oumbn KULnven tor A ttorneys Patented "Nov. 15, 1949 -UNITED STATES PATENT OFFICERADIO COMMUNICATION SYSTEM British company Application July 26, 1947,Serial No. 763,939 In Great Britain May 13, 1946 Claims.

The present invention relates to radio communication systems of the kindin which a transmitted signal is intended to be received by two or morereceivers, the outputs of which operate a common reproducer. Suchcommunication systems may, for example, be used where the transmitter ismobile, in which case a number of receiving stations may be providedwhich are so spaced as to cover the area within which the mobiletransmitter moves, the signal transmitted by the mobile station beingreceived by one or more of the receiving stations, the outputs fromwhich are fed by radio or other links to a main control station whereall the received signals are reproduced on a common producer. With suchsystems, it can happen that the transmitted signal is not received or isonly received with low strength by one or more of the receivingstations, in which case such receivers (which operate with automaticvolume control) will contribute noise to the output from the commonreproducer without contributing any substantial signal and will thustend to reduce the overall signal-to-noise ratio in the signalreproduced by the common reproducer. g

The present invention has for its object to overcome this disadvantageand to provide a system in which the signal-to-noise ratio in the commonreproducer is improved.

To this end, the invention consists in controlling the output of eachreceiver which is fed to the common reproducer inversely in dependenceupon the noise in that receiver relative to a function of the aggregateof the noise in the other receivers or to a function of the approximateaverage noise in all the receivers.

In order that the invention may be more clearly understood, referencewill now be made, by way of example, to the accompanying drawings inwhich:

Fig. 1 shows diagrammatically a communication system of the type towhich the invention applies;

Fig. 2 shows a circuit diagram of two receiver channels feeding a commonreproducer and operating in accordance with this invention.

Referring to Fig. 1, the signals transmitted by the mobile transmitter Tare received by the receiving antennae RI, R2, R3 respectively. Thereceiving station R3 is shown located adjacent the main control station,so that the signals picked up by the station R3 are directly fed to itsassociated receiver B. Stations RI and R2, however, are located remotefrom the main control station S so that the signals picked up therebyand received by the local receivers Al, Cl are retransmitted via radiolinks to corresponding receivers A and C, located in the main controlstation S. The outputs from the three receivers A, B and C in the maincontrol station are fed to the common reproducing device L.

Whilst the receiving station R3 is shown located adjacent the maincontrol station, it will be understood that the main control station maybe remote from all the receiving stations, in which case the signalsreceived by R3 would also be re-transmitted by a radio, link to the maincontrol station.

The three receiving stations RI, R2 and R3 are spacially separated insuch a manner as to cover the entire area within which the mobiletransmitted T is intended to operate, more or less receiving stationsbeing provided depending upon the area to be covered. It will beunderstood, especially where the mobile transmitter T may move over arelatively large area, that the intensity of the signals received by anyone of the receiving stations will depend upon its distance from themobile transmitter, and the automatic volume control circuits of theindividual receivers will operate to increase the gain of the moreremote receivers. Thus, a remote receiving station will produce noise inthe common reproducer L without contributing any significant amount ofsignal intelligence.

According to the present invention, the output from each of thereceivers A, B and C fed to the common reproducer L is controlledinversely in dependence upon the noise in that receiver relative to thenoise in the other or all the receivers.

Fig. 2 shows a receiver circuit diagram for a system with two receiverchannels A and B which operates in accordance with this principle. Thediagram only shows two channels but it will be clear from the followingexplanation how the automatic control connections between the variouschannels may be extended to any number of receiver channels.

Circuit components in channel B, which correspond with the samecomponents in channel A, are indicated by the same reference numerals asare used for channel A but suiiixed with a dash.

As shown in Fig. 2, a portion of the output from the detector oraudio-frequency amplifier stage I of each receiver channel is fed to thegrid of an audio-frequency amplifier valve I4 for amplifying the signalintelligence, the connection preferably being made through a low-passnetwork ii to eliminatethe H. F. noise components. A potential divider19 may be provided for stepping.

down the voltage fed to the valve I 4. The valve 14 is biassed for lessthan maximum gain by the resistors Hi, It connected in its cathodecircuit.

Another portion of the output from the receiver l is fed through ahigh-pass network 2, which transmits the noise components of frequencieshigher than the required intelligence band but does not pass asignificant amount of signal intelligence, to an amplifying valve 3.

In the anode circuit of the valve 3 is connected a transformer 4 havingtwo secondary windings 5 and 6. The voltages developed across thesecondary windings 5 and 6 are rectified by the diodes l and 8respectively and the rectified voltages are developed across loadresistors 9 and I0, shown as potentiometers. These D. C. voltages areproportional to the noise content of the signal in the associatedreceiver channel. A. C. components are substantially eliminated from theoutputs across each load resistor 9, I0, by the shunt capacities H andI2 respectively, or other filters.

The ratio between the two D. C. voltages taken off from the loadresistors 9 and I is adjusted to correspond approximately to the numberof channels in the system, either by appropriate adjustment of the ratioof the two secondary windings and 6 or by adjustment of the position ofthe tapping points on the potentiometers, or by a combination of both ofthese means. the caseof two channels, as shown in the drawing, thevoltage taken from the load resistor 9 (high potential secondarycircuit) is normally adjusted to be approximately twice that taken fromthe load resistor In (low potential secondary circuit). In a similarmanner, in the case of the second channel B, D. C. voltages proportionalto the noise content of the signal from the receiver B are developedacross the potentiometers 9' and H1.

The D. C. voltages taken from the load resistors l0, H1 in the lowpotential secondary circuits are additively connected in series and theextreme negative end of the chain, namely the tapping point on theresistor H1 in the particular circuit shown, is connected to a point ofzero audio-frequency potential, such as to the source of the standingnegative bias for the low frequency amplifying stages carrying theintelligence. The extreme positive end of this chain of resistors in thelow potential secondary circuits is connected to the positive end ofeach of the load resistors 9, 9 in the high potential secondarycircuits. The negative end (tapping point) of each of the load resistors9, 9 in the high potential secondary circuits is connected through a Ddecoupling network 11 to the grid of the corresponding amplifier valvel4, l4 for the signal intelligence associated with the same channel ofthe system.

If it be assumed that the D. C. voltage taken from across the .loadresistor 9 in channel A is Va and the voltage taken from across the loadresistor 9 in channel B is Vb, then, since the voltages taken across thelow potential secondary circuits are, as explained above, approximatelythat proportion of the voltages developed in the high potentialsecondary circuits which corresponds to the number of channels in thesystem the voltages taken from the load resistors I0, H1 in the case ofthe two-channel system under consideration will be Va and 2respectively.

Thus, in

Since, in view of the circuit connections betwen the potentiometersabove described, the voltage obtained from potentiometer 9 is negativeand acts in opposition to the aggregate of the positive voltages derivedfrom the potentiometers l0, ID, the resultant bias applied to the gridof the valve l4 will be:

Similarly in the case of channel B, the bias voltage applied to the gridof the valve 14' will be (VaVb).

More generally,for a larger number of channels n, the voltage applied tothe grid will be given by the expression;

or, alternatively, by the expression:

%(Vb+Vc 2) FromExpression 1 it will be seen that the applied biasvoltage varies inversely as a function of the noise in the channelconcerned relative to the average noise in all the receivers. FromExpression 2 it will be seen that the bias voltage also varies inverselyas a function of the noise in the receiver channel concerned relative toa function of the aggregate of the noise in the other receivers.

The D. C. voltages taken from the resistors 9, 9' are fed throughdecoupling networks ll, H to bias the grids of the signal intelligenceamplifier valves M, l4 respectively. The biassing voltages will vary inaccordance with the above expressions, from which it will be seen that,in the case of channel A, when the noise contents of the two channelsare equal, the gain control biassing voltage applied to valve [4 will bezero. When the noise in channel A is more than that in channel B, theresultant bias will be negative and the gain of the amplifier M will bereduced. Conversely when the noise in channel A is less than that inchannel B, the resultant bias will be positive and the gain of valve Mwill be increased.

Similarly in the case of channel B, the gain of amplifier M will beincreased when the noise in channel B is less than that in channel A andwill be decreased when the noise in channel B is greater than that inchannel A.

The outputs from the two amplifiers I4, I4 are mixed in the mixingamplifier I8 and fed to the common reproducing device L. The finaloutput signal is substantially constant since, when the gain ofamplifier I4 is increased, that of M is decreased and vice versa.

In order to prevent excessive positive voltage being applied to the gridof the amplifier valve H, a diode may be provided with its anodeconnected to the control grid of the valve and its cathode to thejunction of the cathode resistors and it, which point is negative to thepotential of the cathode, so that the negative bias on the Vn- (n-l) Va)grid relative to the cathode is never less than that which permitsmaximum amplification from the amplifier stage.

It will be seen that, with the circuit described, the contribution ofeach channel of the system to the output from the reproducer L isdependent upon the gain of its own audio frequency amplifier M, which isin turn controlled by a voltage inversely proportional to the differencebetween the approximate average of the noise" components in all thechannels on the one hand and the noise components in the particularchannel concerned on the other hand, the gain being increased when thisdifiference is positive and reduced when the difference is negative.

Although a particular embodiment has been described, it will beunderstood that various modifications may be made without departing fromthe scope of the invention as defined by the appended claims.

I claim:

1. Radio communication system comprising at least two receivers, acommon reproducer fed from the outputs of said receivers, means forcontrolling the gain of each of the receivers, means for deriving ineach receiver two D. C. voltages each proportional to the noise contentof the signal received by that receiver, the ratio between the two D. C.voltages being approximately equal to the number of receivers in thesystem, means for producing a D. C. voltage corresponding to thesummation of the lower D. C. voltages produced in each receiver, meansfor combining this summation voltage in opposite sign with the greaterD. C. voltage produced in the said receiver, and means for applying thiscombined voltage to control the gain of the said receiver.

2. Radio communication system as claimed in claim 1, wherein eachreceiver comprises a highpass filter which transmits the noisecomponents of frequencies higher than the frequency intelligence bandbut does not pass a significant amount of signal intelligence, anamplifying valve, means for feeding the signal received by a receiverthrough the high-pass filter to the amplifying valve, a transformerhaving two secondary circuit windings and fed by the output from saidamplifying valve, two rectifiers connected respectively with each ofsaid secondary windings, load resistors connected respectively acrossthe outputs of said rectifiers, and output circuit connections to saidload resistors such that the two D. C. voltages in said circuit outputconnections have a ratio which is approximately proportional to thenumber of receivers in the system.

3. System as claimed in claim 2, wherein the load resistors of each ofthe receivers across which the lower D. C. voltages are derived areconnected in series, the extreme positive end of the chain beingconnected to the positive end of each of the load resistors in each ofthe receivers across which the higher D. C. voltages are derived, theconnections to the negative ends of the load resistors across which thehigher D. C. voltages are derived being connected to bias an amplifyingvalve in the signal amplifying channel of corresponding receiver.

4. System as claimed in claim 2, wherein the ratio between the secondarywindings is approximately proportional to the number of receivers in thesystem.

5. System as claimed in claim 2, wherein the load resistors comprisepotentiometers, the tapping points on whichare adjusted so that voltagesderived from the two potentiometers are approximately proportional tothe number of receivers in the system.

DENIS HAWXBY HUGHES.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,098,286 Garfield Nov. 9, 19372,249,425 Hansell July 15, 1941 2,253,867 Peterson Aug. 26, 19412,290,992 Peterson July 28, 1942 2,303,493 Purrington Dec. 1, 19422,358,448 Earp Sept. 19, 1944 2,379,799 Haigis July 3, 1945 2,384,456Davey Sept. 11, 1945 2,397,830 Bailey Apr. 2, 1946

